The effect of environmental stress on agronomic plants has been a major focus of plant research. Plant productivity is related to the ability of plants to respond to and adapt to environmental stress (Sachs and Ho 1986). The proteins produced by higher plants in response to stress have been well-characterized (Key et al., 1981; Cooper and Ho, 1983; Sachs and Ho, 1986). Many stress proteins have recently been found to be chaperones, a class of proteins involved in the folding of newly synthesized proteins (Ellis and van der Vies, 1991; Gething and Sambrook, 1992; Craig et al., 1993). The chaperones have been proposed to function during stress in binding partially denatured proteins, thus preventing their degradation. Additionally, the chaperones assist in the refolding of these partially denatured proteins into their native structure in an ATP-dependent manner following the relief of stress (Rochester et al., 1986; Ellis and Hemmingsen, 1989; Hendrick and Hartl, 1993; Schroder et al., 1993). The two most extensively studied classes of chaperones are heat shock protein (HSP) 70 homologs and cpn60 homologs.
HSP70 homologs have been found in higher plant cytoplasm (Giorini and Galili, 1991), endoplasmic reticulum (Denecke et al., 1991), chloroplasts (Marshall et al., 1990; Ko et al., 20 1992; Marshall and Keegstra, 1992; Madueno et al., 1993; Wang et al., 1993), and mitochondria (Watts et al., 1992; Neuman et al., 1993). Genes for mitochondrial HSP70 are nuclearly encoded and have been isolated from pea (Watts et al., 1992), potato and tomato (Neuman et al., 1993).
The cpn60s are a group of ubiquitous proteins with a subunit size of approximately 60 kDa that share a functional and structural similarity to the tetradecameric E. coli GroEL complex (Gatenby, 1992). The maize and Arabidopsis thaliana mitochondrial cpn60 genes have been isolated and found to be encoded in the nucleus (Prasad and Stewart, 1992). The maize cpn60 was hypothesized to aid in the assembly of new mitochondrial protein complexes during the rapid organelle biogenesis of seedling germination and heterotrophic growth (Prasad and Stewart, 1992).
Neither the HSP70 homologs nor the cpn 60 homologs are very effective indicators of a heat resistant plant's ability to tolerate heat stress. However, there is another group of heat shock proteins that may be effective indicators. This group is the low molecular mass (17-30 kDa) HSPs (Waters et al., 1996). Recent reports have established that the cytosolic forms of plant small HSPs (sHSPs) can function as molecular chaperones in vitro (Lee et al., 1995a). Lenne and Douce (1994) identified a mitochondrial matrix-localized low molecular mass HSP identified as HSP22. Pea leaf mitochondrial HSP22 is conditionally expressed only at high temperatures and the protein level remained high for at least three days following heat stress (Lenne and Douce, 1994). A cDNA for pea mitochondrial HSP22 has been identified and establishes this protein as a member of the sHSP superfamily (Lenne et al., 1995). cDNAs for mitochondrial sHSPs have also been characterized in soybean (La Fayette et al., 1996), A. thaliana (Willett et al., 1996), and Chenopodium rubrum (Lenne et al., 1995; Waters et al., 1996). However, there has not been an sHSP isolated from a mitochondria of a heat resistant plant. For example, maize is a heat-resistant plant and is one of the world's greatest food sources. The isolation of an sHSP from maize would allow for testing of plants to determine their ability to tolerate heat stress resistance. This testing could take several forms if a sHSP were isolated. Additionally, vectors could be constructed containing the nucleic acid molecule for the sHSP.
There is a need for the discovery of an isolated sHSP protein from a heat resistant plant. This sHSP could be utilized to indicate a heat resistant plant's ability to tolerate heat stress. Additionally, if a sHSP were isolated from a heat resistant plant and cloned into an expression vector, there would be large quantities of HSP for research, the production of antibodies, and the generation of nucleic acid probes.